There is continued and growing interest in the use of renewable resources as replacements for petroleum-derived chemicals. Fatty acid alkyl esters (FAAEs) produced from fats and oils have been investigated as replacements for such petroleum-derived materials, particularly diesel fuel.
It has long been known that triglycerides from fats and oils can be used as fuels for diesel engines. However, such use typically results in engine failure. Remedies for such engine failure wherein conversion of fatty acids, found in lipids, into simple esters, such as methyl and ethyl esters, has been proposed. See, for instance, the process described in U.S. Pat. No. 6,398,707. An increasing body of evidence indicates that these esters perform well in essentially unmodified diesel engines and that such esters may effectively reduce the output of particulate and hydrocarbon pollutants relative to petroleum-diesel fuel. The term “biodiesel” is applied to these esters.
Processes for biodiesel production have been known for many years. For instance U.S. Pat. No. 4,164,506 discloses a biodiesel synthesis wherein fatty acids are subjected to acid catalysis. The conversion of triglycerides with base catalysis is described in U.S. Pat. Nos. 2,383,601 and 2,494,366. Conversion of both free fatty acids and triglycerides with enzyme catalysis is disclosed in U.S. Pat. Nos. 4,956,286, 5,697,986 and 5,713,965. None of these processes, however, completely addresses the production of biodiesel from low value high free fatty acid feedstocks.
An economic analysis of any process for the production of biodiesel indicates that feedstock cost is the largest portion of production cost for biodiesel. Whereas a 15 weight percent free fatty acid (FFA) feedstock is the highest content that any contemporary commercial process has proposed to handle, producers (in order to conserve costs) would prefer to use feedstocks having up to 100 weight percent FFA content.
Further, most of the processes of the prior art are unattractive because they rely upon acid catalyzed esterification of fatty acids. Acid catalysis is not suitable for processing such feedstocks containing FFA concentrations for two principal reasons. First, an excessive amount of acid catalyst is required in order to fully convert feedstocks having high FFA content. Since the acid catalyst must be neutralized before processing the glycerides, the increased catalyst loading results in an excessive amount of generated salt. Further, such processes generate a large volume of waste water as disclosed in U.S. Pat. Nos. 4,303,590, 5,399,731 and 6,399,800.
While enzymatic catalysis has been reported in the literature for esterification of free fatty acids, it is disadvantageous because of reaction product inhibition from the presence of water which results when the free fatty acids in the feedstock are esterified with enzymes. Another problem evidenced from enzymatic processing is the high cost of enzymatic catalysts. Further, enzymatic catalysts have a limited life.
To avoid two-phase operation in packed bed and other reaction settings, some conventional processes for biodiesel production use volatile, toxic co-solvents. Such a process is disclosed in U.S. Pat. No. 6,642,399 B2. The use of volatile, toxic co-solvents is environmentally unacceptable.
Further, some prior art processes for producing biodiesel employ water to wash residual glycerin and salts from the FAAEs. This, however, generates a large volume of wastewater and increases the risk of forming FAAE emulsions, as disclosed in U.S. Pat. No. 5,399,731.
To be economically profitable, the biodiesel industry must take advantage of lower cost feedstocks. Yield is a very important criterion as feedstock costs approach two thirds of the total cost of production of biodiesel. To gain market share in the fuels industry, biodiesel must be competitively priced with conventional hydrocarbon diesel.
Alternative processes need to be developed which do not require high pressures or acid catalysis. In addition, such processes should not employ toxic co-solvents or water for the extraction of impurities. Such processes also need to produce high yield of biodiesel as well as employ inexpensive feedstocks. Further, such feedstocks need to have a high FFA content in order to be competitive with petrodiesel.